NOVEMBER 10 — The recent announcement that the Republic of Korea (South Korea) has secured US approval to develop nuclear-powered submarines marks a significant shift in North-east Asia’s security architecture, with profound implications for Japan and the wider region. This move is not merely bilateral—it signals tectonic shifts in maritime deterrence, alliance burden-sharing, and the evolution of non-proliferation norms in an already volatile Indo-Pacific.

Japan’s strategic dilemma: prestige, principles, and practicalities

For Tokyo, South Korea’s new capability effectively raises a mirror question: if Seoul may now obtain nuclear-propulsion technology for undersea warfare, should Japan ask Washington for the same? Japan is already grappling with mounting threats—from North Korea’s submarine-launched ballistic missiles to China’s expanding undersea fleet.

Yet Japan is bound by its post-war non-nuclear principles—never producing, possessing, or permitting nuclear weapons on its soil—an article of national identity shaped by Hiroshima and Nagasaki. The dilemma is profound: can Japan continue relying on its advanced diesel-electric submarines, or must it follow South Korea into the nuclear-propulsion era?

If Tokyo were to pursue nuclear-powered submarines (SSNs), it would represent a watershed in Japanese defence policy. Such a decision would reinforce the US–Japan alliance technologically but test Japan’s domestic politics and risk regional escalation. A public accustomed to pacifism may resist any move suggesting a drift toward nuclear militarisation.

Alliance management and strategic parity

The US–Japan–South Korea triangle is already delicate. Washington’s green light for Seoul’s SSN ambitions could recalibrate perceptions of alliance equity. Japan may feel its role as Washington’s primary East Asian ally is being diluted, while South Korea gains prestige and operational autonomy.

For Washington, the rationale is pragmatic: integrating South Korea’s formidable shipbuilding industry into America’s maritime supply chain strengthens burden-sharing and offsets US shipyard bottlenecks. But for Japan, it could appear as a redistribution of strategic favour.

If Japan chooses to match Seoul’s move, it must overcome not just legal barriers but the symbolism of its post-war pacifism. Conversely, if it refrains, it risks appearing strategically complacent. Either way, the US decision pressures Tokyo to reassess its own maritime deterrence posture.

Regional implications: The risk of an undersea arms race

The approval for South Korea’s nuclear submarines reverberates far beyond the Korean Peninsula. 

China, Russia, and North Korea are all likely to interpret it as an escalation that justifies further military modernization. Beijing may accelerate its own naval expansion, while Pyongyang could exploit the development to rationalise further nuclear tests or submarine-launched missile deployments.

For South-east Asia, the implications are equally serious. More actors with nuclear-propulsion technology mean greater operational opacity in regional waters like the South China Sea and the East China Sea. Submarine collisions or miscalculations could increase.

Asean states, particularly those maintaining nuclear-weapon-free-zone will be watching carefully. They must balance welcoming a more robust US deterrence posture with fears that the precedent undermines regional denuclearization norms.

Non-proliferation concerns and the US balancing act

While nuclear-powered submarines are not nuclear-armed, the distinction can blur. The transfer of nuclear propulsion technology introduces enriched uranium, reactor design know-how, and sensitive dual-use capabilities. 

Critics argue this could weaken the global non-proliferation regime, especially the Non-Proliferation Treaty (NPT), if more states seek similar exemptions.

The United States thus walks a tightrope—strengthening allied capabilities without eroding its moral authority as a champion of non-proliferation.

Extending SSN access to an ally outside the traditional Aukus framework expands the circle of trust but also expands potential risks. If Japan or Taiwan were to pursue similar capabilities later, the line between “trusted ally” and “new nuclear threshold state” could blur even further.

Japan’s possible strategic responses

Japan has three broad options.

First, it can pursue its own SSN programme under US partnership, leveraging its advanced shipyards and nuclear-energy expertise. 

This would require constitutional reinterpretation, enormous funding, and political courage—yet it would align Japan with South Korea and Australia as full participants in US undersea deterrence.

Second, Japan could maintain its diesel-electric fleet but enhance cooperation with US and South Korean SSNs. Through intelligence-sharing and anti-submarine warfare (ASW) exercises, Japan could preserve strategic parity without crossing its non-nuclear red line.

Third, Tokyo may deepen its leadership in regional arms control, promoting transparency and joint maritime monitoring to prevent accidents and restore balance. By doing so, Japan could reaffirm its moral leadership even as military competition intensifies.

The broader Indo-Pacific context

The South Korean SSN development reflects a broader trend of “nuclear propulsion diffusion” among America’s allies. The precedent of Aukus between the US, UK, and Australia set the stage. 

South Korea’s inclusion, even if outside Aukus, demonstrates Washington’s intent to deepen allied deterrence networks in anticipation of sustained US–China competition.

Japan’s choice will therefore resonate far beyond North-east Asia. Should Tokyo enter the nuclear-sub club, it would signify the normalization of nuclear propulsion in allied navies—potentially inviting India, Indonesia, or even Taiwan to seek similar pathways in the next decade.

The implications for Asean are profound. The region’s waters could become congested with advanced submarines operating at great depth and speed, making strategic stability more fragile. In this environment, maritime confidence-building and communication hotlines will be essential.

A turning point for regional stability

The United States is shifting its deterrence architecture from nuclear exclusivity to allied inclusivity. South Korea’s entry into the SSN circle exemplifies this. 

For Washington, it distributes the burden of deterrence; for Seoul, it enhances strategic autonomy; for Japan, it presents a fundamental choice about its identity and future defence trajectory.

If Japan follows suit, East Asia may witness the most significant naval transformation since the end of World War II. If it does not, Japan must compensate through deeper alliance integration and advanced ASW capabilities. Either way, the balance of power beneath the Pacific’s surface is changing.

Conclusion

The US decision to allow South Korea to build nuclear-powered submarines is a watershed moment in Asia’s maritime order. 

It raises Japan’s stakes in redefining its defence boundaries, recalibrates alliance dynamics, and challenges non-proliferation principles.

As the Indo-Pacific becomes the new frontier of strategic competition, undersea deterrence will define regional power. 

The question now is not whether Japan can join the nuclear-sub club—but whether it must, to remain secure, credible, and relevant in the turbulent waters of the 21st century.

* Phar Kim Beng is Professor of Asean Studies and Director, Institute of International and Asean Studies (IINTAS) International Islamic University Malaysia

** This is the personal opinion of the writer or publication and does not necessarily represent the views of Malay Mail.